摘要：BMP 在發育、生理及病理中均有重要角色。在不同生理情況下，BMP 的作用距離有很大的差異，可想而知，擴散能力的控制是影響BMP 功能的重要因子。但我們目前仍並不甚了解控制BMP 擴散能力的分子機制。本計畫將使用澤蛭胚胎研究此一問題。澤蛭是輪冠動物總門最佳的胚胎學實驗系統，其胚胎發育的主要特色是有固定細胞系譜，可就個別細胞在胚胎中的行為及分化進行觀察。故在實驗分析的時空尺度上，可達到單細胞的解析度。因此，澤蛭胚胎研究可與其他系統互補，探索胚胎發育分子機制的調控及演化。先前的研究顯示，澤蛭神經外胚層腹背體軸生成過程中， BMP5-8 是最關鍵的BMP 分子，但其作用方式並非擴散的morphogen，而是短距離接觸性誘導訊號。本計畫的先導研究建立了於澤蛭胚胎內偵測BMP 作用距離的方法，發現不同細胞型所表現的BMP5-8 有不同作用距離。也發現無法進行前驅蛋白裂解的突變型BMP5-8，仍具有與野生型分子相當的短程誘導能力。這些結果導出以下假說：BMP5-8 前驅蛋白的裂解，控制作用距離長短，且前驅蛋白裂解的差異可造成不同細胞所分泌之BMP5-8有不同擴散模式。本研究將以分子實驗胚胎學、生化分析及分子造影等方法來驗證此一假說。此研究的結果應可對BMP 訊號擴散分子細胞機制的了解做出獨特貢獻。
Abstract: Bone morphogenic protein (BMP) is a family of signaling molecules that play critical roles in developmental, physiological and pathological processes. The deployment and the range of signaling activity of BMPs vary greatly depending on their physiological contexts, and thus the situational control of BMP diffusion is a key element for BMPs to exert their normal functions in a context-dependent manner. However, our knowledge in this area – how BMP diffusion is controlled – is limited. The goal of this project is to identify factor(s) controlling BMP signaling range in the leech Helobdella, one of the best experimental embryology systems in the lophotrochozoan superphylum. The stereotyped cell lineage in the leech embryo allows us to examine the behavior and differentiation of a single identified cell in the whole-embryo context. This feature helps us to achieve a single-cell resolution in the in vivo analysis of spatiotemporal dynamics of developmental mechanisms and makes Helobdella a useful system to complement other major model systems, such as vertebrates and Drosophila, for studying developmental mechanisms and their evolution. The PI’s earlier work has revealed that BMP5-8 is the key BMP molecule in dorsoventral patterning of leech neuroectoderm and that BMP5-8 does not act as a diffusing morphogen but rather as a contact-dependent, short-range inductive signal. In the preliminary study of this project proposal, the PI further established a protocol for detecting BMP5-8 signaling range in Helobdella neuroectoderm and found that BMP5-8 produced by each of the two different cell types in neuroectoderm has a different signaling range. Furthermore, it was also found that, unlike in vertebrate embryos, uncleavable BMP5-8 mutant can function normally in leech neuroectoderm as a dorsally derived short-range inductive signal that specifies dorsal cell fates. Together, these results give rise to the hypothesis that proteolytic processing of BMP5-8 propeptide controls its signaling range in leech neuroectoderm and that cell type-specific proteolytic processing of BMP propeptide results in the observed difference in the signaling ranges of BMP produced by different cell types. This proposal aims to test this hypothesis by using molecular experimental embryology, biochemical and molecular imaging approaches. The results obtained in this project should make some useful contribution to our understanding of how BMP signaling is regulated in different biological contexts.